The invention relates generally to mass storage memory systems, and more particularly, to a method and apparatus for write protecting logical volumes of a mass memory accessible by multiple host computers.
In a typical memory system employing, for example, a disk drive, the entire disk drive can be manually or electronically write protected in advance. For a floppy disk drive, this involves merely flipping a tab, and in a larger disk drive, such as the gigabit drives available today, the write protect can either be manually set at the drive itself, or electronically set by a user or customer engineer. In either case, the drive becomes write protected against all incoming write requests. In addition, the write protection is typically set in advance, so that the status of the drive, that is, what data has been written to the drive, must be known in advance.
Large disk drive memories are typically divided into a plurality of logical volumes. Thus, a single disk drive might have 4, 8, or more logical volumes. In addition, each of the logical volumes can be accessible by plural host computers. Thus, one host computer may be responsible for writing and maintaining a database in a first logical volume on the disk drive and other host computers are ordinarily only expected to read the database for their varying purposes and applications. Other host computers may be responsible for other logical volumes on the same disk drive unit.
In some smart disk drive controllers, such as the EMC Symmetrix disk drive controller, logical volumes were initially protected manually during operation. In these instances, however, the user or customer engineer must know in advance when the write operation(s) for data on the drive which is to be protected, has been completed, and thereafter, the user or customer engineer would write protect the logical volume or the entire drive manually.
In our copending U.S. patent application Ser. No. 08/831,481, filed on Mar. 31, 1997, and entitled DISK ARRAY WRITE PROTECTION AT THE SUB-UNIT LEVEL, the contents of which are incorporated herein by reference, there is described a write protect mechanism operable from the host computer. This invention is an improvement upon the invention described in our earlier copending application.